Curable, weldable coating compositions

ABSTRACT

A curable coating composition is disclosed comprising a resinous binder comprising (a) a reaction product of an epoxy-containing polymer with a compound containing phosphorus acid groups, the reaction product having reactive functional groups, and (b) a curing agent having functional groups reactive with the functional groups of (a). An electroconductive pigment is dispersed in (a) such that the weight ratio of the electroconductive pigment to (a) plus (b) is within the range of 0.5 to 9.0:1. When the curable coating composition is deposited and cured on a metal substrate, the cured coating is weldable.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/267,521, filed Feb. 8, 2001.

FIELD OF THE INVENTION

[0002] This invention relates generally to curable, weldable coatingsfor metal substrates, and more particularly, to curable, weldablecoatings for metal substrates, which inhibit corrosion.

[0003] BACKGROUND OF THE INVENTION

[0004] The production of light gauge steel for end uses ranging fromarchitectural construction materials to automobiles is well known. Arolling mill line produces continuous sheets of steel in the requiredthickness and width. The steel sheets may be coated with a thin layer ofzinc metal via a galvanizing process. Eventually, mill oil is applied tothe uncoated or galvanized steel sheets, and the steel is either storedor shipped in a coil to a customer for further processing.

[0005] Typically, the customer is an automobile manufacturer who willtake the coiled metal sheet and pass it through a lubricating stationand then to a forming operation where the metal sheet is cut and formedinto a part such as a roof, fender, door, etc. The parts are then weldedtogether to form an automobile body. Next, the automobile body iscleaned, treated with a zinc phosphating solution to enhance corrosionprotection, and rinsed with deionized water. The treated automobile bodyis then passed through an electrodeposition bath where a corrosionresistant primer is applied.

[0006] The automobile manufacturers would like to streamline theiroperations and have some of the operations described above done outsidethe automobile assembly plant, for example at a steel mill or a customcoater. One major problem with moving certain operations to a steel millor a custom coater is that any coating applied outside the automobileassembly plant must be able to accept a weld. At some point in time, thevarious metal parts will be welded together in the automobile assemblyplant to form the automobile body. Consequently, automobilemanufacturers have a strong demand for a weldable, corrosion resistantcoating composition that can be applied at a steel mill or at a customcoating facility.

[0007] Such a weldable, corrosion resistant coating composition could beapplied at a custom coater, known as a coil coater, who would ship thecoated metal sheet to the automobile manufacturer. As described above,the automobile manufacturer would then form the metal sheet into partsand weld the parts together. However, the metal pretreatment operationand perhaps the electrodeposition process could be avoided since themetal received by the automobile manufacturer would already contain acorrosion resistant coating.

[0008] Similar to the above, a weldable, corrosion resistant coatingcomposition could also be applied at a steel mill. Application at thesteel mill enables the automobile manufacturer to receive corrosionresistant metal directly without the expense associated with shippingthe metal to a coil coater and from the coil coater to the automobilemanufacturer.

[0009] The present invention provides a weldable, curable coatingcomposition that provides corrosion protection and can be applied by acoil coater or at a steel mill, can be cured at low temperature andprovides good adhesion and good corrosion protection without prior metalpretreatment.

SUMMARY OF THE INVENTION

[0010] One aspect of the present invention is a curable coatingcomposition comprising:

[0011] a. a resinous binder comprising:

[0012] i. a reaction product of an epoxy-containing polymer with acompound containing phosphorus acid groups, the reaction product havingreactive functional groups,

[0013] ii. a curing agent having functional groups reactive with thefunctional groups of (i);

[0014] b. an electroconductive pigment dispersed in (a) such that theweight ratio of b to (i) plus (ii) is within the range of 0.5 to 9.0:1,the curable coating composition being characterized such that when it isdeposited and cured on a metal substrate, the cured coating is weldable.

[0015] Another aspect of the present invention is an aqueous-basedcoating composition comprising:

[0016] a. a resinous binder comprising:

[0017] i. a reaction product of an epoxy-containing polymer with acompound containing phosphorus acid groups, the reaction product havingreactive functional groups,

[0018] ii. a curing agent having functional groups reactive with thefunctional groups of (i);

[0019] b. an electroconductive pigment dispersed in (a) such that theweight ratio of b to (i) plus (ii) is within the range of 0.5 to 9.0:1;and:

[0020] c. water, the coating composition being characterized such thatwhen it is deposited and cured on a metal substrate, the cured coatingis weldable.

[0021] Yet, another aspect of the present invention is an organicsolvent-based coating composition comprising:

[0022] a. a resinous binder comprising:

[0023] i. a reaction product of an epoxy-containing polymer with acompound containing phosphorus acid groups, the reaction product havingreactive functional groups,

[0024] ii. a curing agent having functional groups reactive with thefunctional groups of (i);

[0025] b. an electroconductive pigment dispersed in (a) such that theweight ratio of b to (i) plus (ii) is within the range of 0.5 to 9.0:1;and

[0026] c. an organic solvent, the curable coating composition beingcharacterized such that when it is deposited cured on a metal substrate,the cured coating is weldable.

[0027] Another aspect of the present invention is a process for coatinga continuous strip or coil of metal comprising:

[0028] a. applying directly to the metal sheet'shortly after it isformed and at a temperature of 20 to 150° C., a curable coatingcomposition comprising:

[0029] i. a resinous binder comprising

[0030] (A) a reaction product of an epoxy-containing polymer with acompound containing phosphorus acid groups, the reaction product havingreactive functional groups,

[0031] (B) a curing agent having functional groups reactive with thefunctional groups of (A);

[0032] ii. an electroconductive pigment dispersed in (i) such that the

[0033] weight ratio of (ii) to (A) plus (B) is within the range of 0.5to 9.0:1,

[0034] the curable coating composition being characterized such thatwhen it is deposited and cured on a metal substrate, the cured coatingis weldable; and

[0035] b. drying the coating composition on the metal sheet.

[0036] Yet, another aspect of the invention is a process for coating acontinuous metal coil comprising:

[0037] a. unwinding the metal sheet from a metal coil and passing themetal sheet in a substantially continuous manner through a cleaningstation, a coating station, and a curing station;

[0038] b. applying to the metal sheet at the coating station a curablecoating composition comprising:

[0039] i. a resinous binder comprising:

[0040] (A) a reaction product of an epoxy-containing polymer with aphosphorus-containing acid, the reaction product having reactivefunctional groups,

[0041] (B) a curing agent having functional groups reactive with thefunctional groups of (A);

[0042] ii. an electroconductive pigment dispersed in (i) such that theweight ratio of (ii) to (A) plus (B) is within the range of 0.5 to9.0:1; and

[0043] c. curing the coating composition applied to the metal sheet instep (b) as the coated metal sheet passes through the curing station.

Detailed Description of the Preferred Embodiments

[0044] The use of numerical values in the various ranges specified inthis application, unless expressly indicated otherwise, are stated asapproximations as though the minimum and maximum values within thestated ranges are both preceded by the word “about”.In this manner,slight variations above and below the stated ranges can be used toachieve substantially the same results as values within the ranges.Also, the disclosure of these ranges is intended-as a continuous rangeincluding every value between the minimum and maximum values.

[0045] The present invention is a curable coating composition for metalsubstrates that can be applied without pretreatment and methodsinvolving the same. The curable coating composition comprises a resinousbinder. The resinous binder comprises a reaction product of anepoxy-containing polymer with a compound containing phosphorus acidgroups. The reaction product has reactive functional groups.

[0046] Useful epoxy-containing polymers have at least one epoxy oroxirane group in the molecule, such as polyglycidyl ethers of polyhydricalcohols. Useful polyglycidyl ethers of polyhydric alcohols can beformed by reacting epihalohydrins like epibromohydrin, dichlorohydrinand epichlorohydrin with polyhydric alcohols, such as dihydric alcohols,in the presence of an alkali condensation and dehydrohalogenationcatalyst. Suitable alkali condensation and dehydrohalogenation catalystinclude sodium hydroxide or potassium hydroxide.

[0047] Suitable polyhydric alcohols can be aromatic, aliphatic orcycloaliphatic. Non-limiting examples of suitable aromatic polyhydricalcohols include phenols that are preferably at least dihydric phenols.Other useful aromatic polyhydric alcohols include dihydroxybenzenes, forexample resorcinol, pyrocatechol and hydroquinone;bis(4-hydroxyphenyl)-1,1-isobutane; 4,4-dihydroxybenzophenone;bis(4-hydroxyphenyl)-1,1-ethane; bis(2-hydroxyphenyl)methane;1,5-hydroxynaphthalene; 4-isopropylidene bis(2,6-dibromophenol);1,1,2,2-tetra(p-hydroxy phenyl)-ethane; 1,1,3-tris(p-hydroxyphenyl)-propane; novolac resins; bisphenol F; long-chain bisphenols; and2,2-bis(4-hydroxyphenyl)propane, i.e., bisphenol A.

[0048] Non-limiting examples of aliphatic polyhydric alcohols includeglycols such as ethylene glycol, diethylene glycol, triethylene glycol,1,2-propylene glycol, 1,4-butylene glycol, 2,3-butylene glycol,pentamethylene glycol, polyoxyalkylene glycol; polyols such as sorbitol,glycerol, 1,2,6-hexanetriol, erythritol and trimethylolpropane; andmixtures thereof. An example of a suitable cycloaliphatic alcohol iscyclohexanedimethanol.

[0049] Preferably, the epoxy-containing polymer has at least two epoxygroups per molecule and aromatic or cycloaliphatic functionality toimprove adhesion to a metal substrate. It is also preferred that theepoxy-containing polymer be relatively more hydrophobic than hydrophilicin nature. Further, the epoxy-containing polymer should have a numberaverage molecular weight of about 220 to 25,000. The molecular weightcan be determined by multiplying the epoxy equivalent weight or epoxyequivalent by the epoxy functionality or number of epoxy groups.

[0050] Useful epoxy-containing polymers are disclosed in U.S. Pat. Nos.5,294,265; 5,306,526 and 5,653,823, which are hereby incorporated byreference. Other useful epoxy-containing materials includeepoxy-functional acrylic polymers, glycidyl esters of carboxylic acidsand mixtures thereof. Examples of suitable commercially availableepoxy-containing polymers are available from Shell Chemical Companyunder the trademarks EPON® 836, EPON® 828, EPON® 1002F and EPON® 1004F.EPON® 836 and EPON® 828 are epoxy functional polyglycidyl ethers ofbisphenol A prepared from bisphenol A and epichlorohydrin. EPON® 828 hasa number average molecular weight of about 400 and an epoxy equivalentweight of about 185-192. EPON® 836 has an epoxy equivalent weight ofabout 178-186.

[0051] The compound containing phosphorus acid groups that is reactedwith the epoxy-containing polymer comprises phosphonic acids,phosphorous acid, phosphoric acids (preferred) including super- andpoly-, and mixtures thereof.

[0052] Examples of suitable phosphonic acids include those having atleast one group of the structure:

—R—PO—(OH)₂

[0053] where R is -C-, preferably CH₂, and more preferably O—CO—(CH₂)₂—.Nonlimiting examples of suitable phosphonic acids include1-hydroxyethylidene-1,1-diphosphonic acid, methylene phosphonic acids,and alpha-aminomethylene phosphonic acids containing at least one groupof the structure:

[0054] such as (2-hydroxyethyl)aminobis(methylene phosphonic) acid,isopropylaminobis(methylenephosphonic) acid and other aminomethylenephosphonic acids disclosed in U.S. Pat. No. 5,034,556 at column 2, line52 to column 3, line 43, which is hereby incorporated by reference.

[0055] Other useful phosphonic acids include alpha-carboxymethylenephosphonic acids containing at least one group of the structure:

[0056] Nonlimiting examples of suitable phosphonic acids includebenzylaminobis(methylene phosphonic) acid, cocoaminobis(methylenephosphonic) acid, triethylsilylpropylamino(methylene phosphonic) acidand carboxyethyl phosphonic acid.

[0057] The equivalent ratio of the compound containing phosphorus acidgroups to epoxy-containing polymer is within the range of 0.3 to 5.0:1,preferably 0.5 to 3.5:1. The epoxy-containing polymer and thecompound-containing phosphorus acid groups can be reacted together byany method known to those skilled in the art.

[0058] The functional groups associated with the reaction product of theepoxy-containing polymer and the compound-containing phosphorus acidgroups are hydroxyl groups including acidic hydroxyls or hydroxyl groupsand epoxy groups depending on the equivalent ratio of the compoundcontaining phosphorus acid groups to epoxy-containing polymer.

[0059] The resinous binder of the present invention also comprises acuring agent having functional groups that are reactive with thefunctional groups of the reaction product described above. The curingagent can be selected from aminoplasts, polyisocyanates, includingblocked isdcyanates, polyacids, organometallic acid-functionalmaterials, polyamines, polyamides and mixtures of any of the foregoingdepending on the identity of the functional groups in the reactionproduct.

[0060] Useful aminoplasts can be obtained from the condensation reactionof formaldehyde with an amine or amide. Nonlimiting examples of aminesor amides include melamine, urea and benzoguanamine.

[0061] Although condensation products obtained from the reaction ofalcohols and formaldehyde with melamine, urea or benzoguanamine are mostcommon, condensates with other amines or amides can be used. Forexample, aldehyde condensates of glycoluril, which yield a high meltingcrystalline product useful in powder coatings, can be used. Formaldehydeis the most commonly used aldehyde, but other aldehydes such asacetaldehyde, crotonaldehyde, and benzaldehyde can also be used.

[0062] The aminoplast can contain imino and methylol groups. In certaininstances, at least a portion of the methylol groups can be etherifiedwith an alcohol to modify the cure response. Any monohydric alcohol likemethanol, ethanol, n-butyl alcohol, isobutanol, and hexanol can beemployed for this purpose. Nonlimiting examples of suitable aminoplastresins are commercially available from Cytec Industries, Inc. under thetrademark CYMEL® and from Solutia, Inc. under the trademark RESIMENE®.Preferred aminoplasts are CYMEL® 385 (preferred for water-basedcompositions), CYMEL® 1158 imino-functional melamine formaldehydecondensates,-and CYMEL® 303.

[0063] Other curing agents suitable for use include, but are not limitedto, polyisocyanate curing agents. As-used herein, the term“polyisocyanate” is intended to include blocked (or capped)polyisocyanates as well as unblocked polyisocyanates. The polyisocyanatecan be aliphatic, aromatic, or a mixture of the foregoing. Althoughhigher polyisocyanates such as isocyanurates of diisocyanates are oftenused, diisocyanates can be used. Higher polyisocyanates also can be usedin combination with diisocyanates. Isocyanate prepolymers, for examplereaction products of polyisocyanates with polyols also can be used.Mixtures of polyisocyanate curing agents can be used.

[0064] If the polyisocyanate is blocked or capped, any suitablealiphatic, cycloaliphatic, or aromatic alkyl monoalcohol known to thoseskilled in the art can be used as a capping agent for thepolyisocyanate. Other suitable capping agents include oximes andlactams. Other useful curing agents comprise blocked polyisocyanatecompounds such as, for example the tricarbamoyl triazine compoundsdescribed in detail in U.S. Pat. No. 5,084,541, which is incorporatedherein by reference.

[0065] Suitable curing agents are described in U.S. Pat. No. 4,346,143at column 5, lines 45-62 and include blocked or unblocked di- orpolyisocyanates such as toluene diisocyanate blocked with caprolactam. Atoluene diisocyanate blocked with caprolactam is commercially availablefrom Bayer Corporation under the trademark DESMODUR® BL 1265.

[0066] Suitable polyacid curing agents include acid group-containingacrylic polymers prepared from an ethylenically unsaturated monomercontaining at least one carboxylic acid group and at least oneethylenically unsaturated monomer that is free from carboxylic acidgroups. Such acid functional acrylic polymers can have an acid numberranging from 30 to 150. Acid functional group-containing polyesters canbe used as well. The above-described polyacid curing agents aredescribed in further detail in U.S. Pat. No. 4,681,811 at column 6, line45 to column 9, line 54, which is incorporated herein by reference.

[0067] Useful organometallic complexed materials which can be used ascuring agents include a stabilized ammonium zirconium carbonate solutioncommercially available from Magnesium Elektron, Inc. under the trademarkBACOTE™ 20, stabilized ammonium, zirconium carbonate, and a zinc-basedpolymer crosslinking agent commercially available from Ultra AdditivesIncorporated under the trademark ZINPLEX 15.

[0068] Nonlimiting examples of suitable polyamine curing agents includeprimary or secondary diamines or polyamines in which the radicalsattached to the nitrogen atoms can be saturated or unsaturated,aliphatic, alicyclic, aromatic, aromatic-substituted-aliphatic,aliphatic-substituted-aromatic, and heterocyclic. Nonlimiting examplesof suitable aliphatic and alicyclic diamines include 1,2-ethylenediamine, 1,2-propylene diamine, 1,8-octane diamine, isophorone diamine,propane-2,2-cyclohexyl amine, and the like. Nonlimiting examples ofsuitable aromatic diamines include phenylene diamines and toluenediamines, for example o-phenylene diamine and p-tolylene diamine. Theseand other suitable polyamines are described in detail in U.S. Pat. No.4,046,729 at column 6, line 61 to column 7, line 26, which isincorporated herein by reference.

[0069] Appropriate mixtures of curing agents may also be used in theinvention. The weight percent of the curing agent generally ranges from5 to 60 percent based on the total weight of the resinous binder.

[0070] The curable coating also comprises an electroconductive pigmentdispersed in the resinous binder. Nonlimiting examples of suitableelectroconductive pigments include zinc, aluminum, iron, graphite, ironphosphide, tungsten, stainless steel, and mixtures thereof. Suitablezinc pigments are commercially available from ZINCOLI GmbH under thetrademark ZINCOLIS 620 or 520. Suitable iron phosphide pigments arecommercially available from Occidental Chemical Corporation under thetrademark FERROPHOS™.

[0071] The electroconductive pigment is dispersed in the resinous bindersuch that the curable coating composition deposited and cured on a metalsubstrate is weldable. The term “weldable” is defined as beingsufficiently electroconductive to sustain a spot welding and joiningoperation as used in an automotive assembly plant. Preferably, theweight ratio of the electro-conductive pigment to the reaction productplus curing agent is within the range of 0.2 to 10. Also, the weightpercent of electroconductive pigment based on the total weight ofresinous binder plus electroconductive pigment is from 30 to 95 percent.

[0072] The curable coating composition may contain a diluent. Diluentsare is added to adjust the viscosity of the coating composition. If adiluent is used, it should not detrimentally affect the adhesion of thecurable coating composition to the metal substrate. Useful diluentsinclude water, organic solvents, or mixtures of water and organicsolvents.

[0073] When water is included as a diluent, dispersants, thickeners,stabilizers, rheology modifiers, and anti-settling agents are required.A suitable rheology-modifier is available from Rohm and Haas Companyunder the trademark Rheology Modifier QR-708, Experimental. A suitablestabilizing and dispersing agent is potassium tripolyphosphate (KTPP).When prepared, the viscosity of the aqueous composition is 300-12,000 cp(Brookfield Cone and Plate). When the composition is shipped, it is upto 35 percent water by weight with a viscosity of about 100-2000 cp. Atapplication, the composition will be no more than 50 percent water byweight with a viscosity between 20-100 cp.

[0074] Optimally, the aqueous composition will contain an amine. Thepreferred amines are hydroxyl-containing amines. The volatile organiccompound content (VOC content) of the aqueous composition will be lessthan 2, preferably less than 1.7.

[0075] Method 24 is a common method for determining VOC content.According to Method 24, the VOC content for single component coatings isdetermined by calculating the total volatile content in grams for thewater and/or exempt material content in grams and dividing by the volumeof the test specimen corrected for the water and/or exempt materialvolume. The VOC content is reported as the mass per unit volume ofcoating (grams per liter or pounds per gallon) or as the mass per unitvolume of coating solids (grams per liter of solids).

[0076] For multi-component coatings, the VOC content is determined usingthe following equations: ${VOC} = \frac{\begin{matrix}\left( {{total}\quad {volatiles}\quad {less}\quad {water}\quad {less}\quad {exempt}\quad {solvents}} \right) \\\left( {{density}\quad {of}\quad {coating}} \right)\end{matrix}}{\begin{matrix}{{100\%} - \left( {{volume}\quad {percent}\quad {of}\quad {water}} \right) -} \\\left( {{voulme}\quad {percent}\quad {of}\quad {exempt}\quad {solvents}} \right)\end{matrix}}$ or${VOC} = \frac{({Wo})({Dc})}{{100\%} - {Vw} - {Vex}}$

[0077] Where:

[0078] W_(o)=weight percent of organic volatiles

[0079] V_(W)=volume of water, %, (W_(w))(D_(c)/D_(w))

[0080] V_(ex)=volume of exempt solvent, %, (W_(ex))(D_(c)/D_(ex))

[0081] D_(c)=density of coating, g/L, at 25° C.

[0082] The VOC content for multi-component coatings is expressed as themass of VOC per unit volume of the coating minus water and exemptsolvents.

[0083] The diluent of the present invention can be an organic solvent.Suitable organic solvents include alcohols having up to about 8 carbonatoms, such as ethanol and isopropanol; and alkyl ethers of glycols,such as 1-methoxy-2-propanol, and monoalkyl ethers of ethylene glycol,diethylene glycol and propylene glycol. Preferably, the diluent includesa propylene glycol monomethyl ether or a dipropylene glycol monomethylether that are commercially available from Dow Chemical Company. Asuitable propylene glycol monomethyl ether is available from DowChemical Company under the trademark DOWANOL PM. A suitable dipropyleneglycol monomethyl ether is commercially available under the trademarkDOWANOL DPM.

[0084] Other suitable organic solvents include ketones such ascyclohexanone (preferred), acetone, methyl ethyl ketone, methyl isobutylketone and isophorone; esters and ethers such as 2-ethoxyethyl acetate,propylene glycol methyl ether acetates such as PM ACETATE, which iscommercially available from Dow Chemical Company; and aromatic solventssuch as toluene, xylene, aromatic solvent blends derived from petroleumsuch as those available under the trademark SOLVESSO®.

[0085] When prepared, the viscosity of the organic solvent-containingcomposition is 300-12,000 cp (Brookfield Cone and Plate). When thecomposition is shipped, it is 20-40 percent organic solvent by weightwith a viscosity of about 100-2000 cp. At application, the compositionwill be approximately 50 percent organic solvent by weight, thecomposition with a viscosity between 20-100 cp.

[0086] The solvent-based composition contains an amine for stabilitypurposes. The preferred amines are alkyl substituted morpholinecompounds such as N-methyl and N-ethyl morpholine.

[0087] Optimally, the curable coating composition of the invention canfurther comprise surfactants. Surfactants can be used to improve thewetting of the substrate. Generally, surfactants are present in anamount of less than about 2 weight percent on a basis of total weight ofthe coating composition. Suitable surfactants are commercially availablefrom Air Products and Chemicals, Inc. under the trademark SURFYNOL 104PA.

[0088] The coating composition of the present invention can also includecorrosion resistant pigments. Suitable corrosion resistant pigmentsinclude, but are not limited to, zinc phosphate, calcium ion-exchangedsilica, colloidal silica, synthetic amorphous silica, and molybdatessuch as calcium molybdate, zinc molybdate, barium molybdate, strontiummolybdate, and mixtures thereof. Suitable calcium ion-exchanged silicais commercially available from W.R. Grace & Co. under the trademarkSHIELDEX® AC3. Suitable colloidal silica is available from NissanChemical Industries, Ltd. under the trademark SNOWTEX. Suitableamorphous silica is available from W.R. Grace & Co. under the trademarkSYLOID®.

[0089] The curable coating composition can further comprise otheroptional ingredients such as inorganic lubricants like molybdenumdisulfide particles that are commercially available from ClimaxMolybdenum Marketing Corporation. The coating composition can alsoinclude extender pigments such as iron oxides and iron phosphides, flowcontrol agents, and thixotropic agents such as silica, montmorilloniteclay and hydrogenated castor oil. Further, the coating composition caninclude anti-settling agents such as aluminum stearate and polyethylenepowder, dehydrating agents which inhibit gas formation such as silica,lime or sodium aluminum silicate, and wetting agents including salts ofsulfated castor oil derivatives such as those commercially availablefrom Cognis Corporation under the trademark RILANIT R4.

[0090] Preferably, the curable coating composition is essentially freeof chromium-containing materials, i.e., contains less than about 2weight percent of chromium-containing materials (expressed as CrO₃), andmore preferably less than about 0.05 weight percent ofchromium-containing materials, and most preferably about 0.00001 weightpercent. Examples of such chromium-containing materials include chromicacid, chromium trioxide, chromic acid anhydride, dichromate salts suchas ammonium dichromate, sodium dichromate, potassium dichromate, andcalcium chromate.

[0091] In practice, the curable coating composition of the presentinvention will be applied on a metal substrate. Metal substrates used inthe practice of the present invention include ferrous metals,non-ferrous metals and combinations thereof. Suitable ferrous metalsinclude iron, steel, and alloys thereof. Nonlimiting examples of usefulsteel materials include cold rolled steel, galvanized (zinc coated)steel, electrogalvanized steel, stainless steel, pickled steel,zinc-iron alloy such as Galvanneal, Galvalume and Galfan zinc-aluminumalloys and combinations thereof. Useful non-ferrous metals includealuminum, zinc, magnesium and alloys thereof. Combinations or compositesof ferrous and non-ferrous metals can also be used.

[0092] At application, the temperature of the coating composition istypically about 10° C. to about 85° C., and preferably about 15° C. toabout 60° C. For aqueous coating compositions, the pH of the coatingcomposition at application generally ranges from about 7.0 to about12.0, and is preferably about 8.0 to about 10.5.

[0093] If the pH of the coating composition needs to be adjusted,water-soluble or water-dispersible acids and/or bases can be used.Suitable acids include mineral acids, such as hydrofluoric acid,fluoroboric acid, phosphoric acid, and nitric acid; organic acids, suchas lactic acid, acetic acid, hydroxyacetic acid, citric acid, andmixtures thereof. Suitable bases include inorganic bases, such as sodiumhydroxide and potassium hydroxide; nitrogen-containing compounds such asammonia, triethylamine, methyl ethanol amine, diisopropanolamine; andmixtures thereof.

[0094] The curable coating composition of the invention can be appliedto the surface of a metal substrate by any conventional applicationtechnique, such as spraying, immersion or roll coating in a batch orcontinuous process. Squeegee or wringer rolls can be used to removeexcess coating. After application, the curable coating is cured to forma cured coating upon the metal substrate. Curing can be achieved at peakmetal temperatures of 100-400° C. Peak metal temperatures of about 150°C. to about 300° C. are preferred. The cure times utilized in thepresent invention range from twenty (20) seconds to sixty (60) minutes.

[0095] The thickness of the applied coating is determined mainly by theapplication conditions. Generally, to achieve sufficient corrosionresistance for automotive use, the applied coating should have a filmthickness of at least about 1 micrometer (about 0.04 mils), preferablyabout 1 to about 20 micrometers, and more preferably about 2 to about 10micrometers. For other substrates and other applications, thinner orthicker coatings can be used.

[0096] One of the major advantages of the curable coating composition ofthe invention is that it can be applied either at a steel mill or a coilcoating facility. When the coating composition is applied at a steelmill, the following steps are followed. First, the curable coatingcomposition is applied directly to a metal sheet shortly after it isformed and at a temperature of 20 to 150° C. Second, the coatingcomposition is dried using an IR oven. IR ovens generate the high peakmetal temperatures in short periods of time (2 to 30 seconds).

[0097] When the coating composition is applied at a coil coatingfacility, the process is as follows. A metal sheet is unwound from ametal coil and passed through a cleaning station, a coating station, anda curing station in a substantially continuous manner. As the metalsheet passes through the coating station, the curable coatingcomposition of the present invention is applied to the metal sheet. Thecoating composition is cured as the coated metal sheet passes throughthe curing station.

[0098] The present invention will now be illustrated by the followingspecific, non-limiting examples. All parts and percentages are by weightunless otherwise indicated.

EXAMPLES

[0099] The following Examples are included in the application todescribe and highlight the present invention. Examples A-F show how theresin of the present invention is synthesized. Examples 1-5 illustratespecific formulations of the coating composition according to thepresent invention. The Examples include a section that describes thepreparation and subsequent coating of substrates according to thepresent invention. The Examples also include a section that shows theperformance of substrates coated with the coating composition accordingto the present invention in regards to adhesion and corrosion.

RESIN SYNTHESES Example A

[0100] To a 4-neck 3-liter round-bottom flask fitted with areflux-condenser, a mechanical stirrer and a nitrogen inlet, werecharged at ambient temperature 36.9 grams (0.32 mole) of 85% phosphoricacid and 50 grams of DOWANOL PM. The mixture was heated with stirring to99° C. while maintaining a nitrogen blanket. A'solution comprising 554grams (0.6 mole) of EPON01004F commercially available from ShellChemical Company and 553 grams of DOWANOL PM was added to the flask froman addition funnel at 99-100° C. over 52 minutes. The reaction mixturewas then held at 100° C. for 53 minutes at which time the epoxyequivalent weight was determined to be greater than 20,000. Next, 21.6grams of deionized water were added and the reaction mixture was held at100-104° C. for 123 minutes. The reaction mixture was then cooled to 82°C., and a vacuum was applied resulting in 253 grams of distillateremoved. To the reaction mixture were then added 57 grams (0.64 moles)of dimethylethanol amine dissolved in 100 grams of deionized water overeight minutes at 82° C. After mixing well, 934.5 grams of deionizedwater (preheated to approximately 70° C.) were added to the reactionmixture at 72-57° C. over 30 minutes. The reaction mixture was thencooled and poured into a plastic container. The solids of the resinsolution were determined to be 31.1%, and the acid number was determinedto be 18.1.

Example B

[0101] To a 4-neck 5-liter round-bottom flask fitted with a refluxcondenser, a mechanical-stirrer and a nitrogen inlet were charged atambient temperature 1880 grams (5.0 moles) of EPON® 828, 684 grams (3.0moles) of Bisphenol A and 2.6 grams of ethyltriphenylphosphonium iodide.The mixture was stirred and heated to 130° C. while maintaining anitrogen blanket. The reaction mixture was allowed to exotherm andreached a maximum temperature of 173° C. The reaction mixture was thenheld for about one hour as the temperature was allowed to fall to 150°C. The reaction mixture was then cooled to 120° C. over 60 minutes. Thereaction mixture was then diluted by the addition of 1100 grams ofDOWANOL PM over 35 minutes. The reaction mixture was then cooled andpoured into a metal container and designated “epoxy resin solutionX”.The solids of the resin solution were determined to be 70.9%, and theepoxy equivalent weight was determined to be 917 as measured bypotentiometric titration.

Example C

[0102] To a 4-neck 5-liter round-bottom flask fitted with a refluxcondenser, a mechanical stirrer and a nitrogen inlet were charged atambient temperature 123.1 grams (1.067 moles) of 85% phosphoric acid and200 grams of DOWANOL PM. The mixture was stirred and heated to 99° C.while maintaining a nitrogen blanket. A solution comprising 1834 grams(1.0 mole) of “epoxy resin solution X” and an additional 519.7 grams ofDOWANOL PM were added to the flask from an addition funnel at 99° C.over 78 minutes. An additional 100 grams of DOWANOL PM were used as arinse for the addition funnel. The rinse was added to the reactionmixture. The reaction mixture was then held at 99° C. for 59 minutes atwhich time the epoxy equivalent weight was determined to be greater than20,000 as measured by potentiometric titration. The reaction mixture wasthen cooled and filled out into a plastic container. The solids of theresin solution were determined to be 55.6%, and the acid number wasdetermined to be 40.1.

Example D

[0103] To a 4-neck 5-liter round-bottom flask fitted with a refluxcondenser, a mechanical stirrer and a nitrogen inlet were charged atambient temperature 1880 grams (5.0 moles) of EPON® 828, 684 grams (3.0moles) of Bisphenol A and 2.6 grams of ethyltriphenylphosphonium iodide.The mixture was stirred and heated to 130° C. while maintaining anitrogen blanket. The reaction mixture was allowed to exotherm andreached a maximum temperature of 172° C. The reaction mixture was thenheated to 180° C. and held for about one hour at 180° C. The heat wasturned off, and the reaction mixture was allowed to stand overnightwhile maintaining a nitrogen blanket. The next morning, heat wascarefully applied to melt the resin and when the resin was partiallymelted, 1100 grams of DOWANOL PM were added. The reaction mixture wasthen heated with good mixing until all the resin was dissolved. Theresin solution was cooled and filled out into a metal container anddesignated “epoxy resin solution Y”.The solids of the resin solutionwere determined to be 69.8%, and the epoxy equivalent weight wasdetermined to be 945.

Example E

[0104] To a 4-neck 5-liter round-bottom flask fitted with a refluxcondenser, a mechanical stirrer and a nitrogen inlet, were charged atambient temperature 47.5 grams (0.267 mole) of superphosphoric acid and221.3 grams of DOWANOL PM. The mixture was heated with stirring to 89°C. while maintaining a nitrogen blanket. A solution comprising 945 grams(0.5 mole) of “epoxy resin solution Y” and an additional 154.2 grams ofDOWANOL PM were added to the flask from an addition funnel at 89-90° C.over 54 minutes. An additional 50 grams of DOWANOL PM were used as arinse for the addition funnel. The rinse was added to the reactionmixture. The reaction mixture was then held at 90° C. for about one hourat which time the heat was turned off and the resin solution allowed tostand overnight while maintaining a nitrogen blanket. The next morningthe reaction mixture was heated to 89° C. and the epoxy equivalentweight was determined to be greater than 20,000. The reaction mixturewas then cooled and filled out into a plastic container. The solids ofthe resin solution were determined to be 51.4%, and the acid number wasdetermined to be 28.3.

Example F

[0105] To a 4-neck 3-liter round-bottom flask fitted with a refluxcondenser, a mechanical stirrer and a nitrogen inlet were charged atambient temperature 888 grams (0.5 mole) of EPON® 1004F and 832 grams ofDOWANOL PM. The mixture was heated with stirring to 101° C. whilemaintaining a nitrogen blanket. A solution comprising 47.5 grams (0.267mole) of superphosphoric acid and 47.5 grams of DOWANOL PM were addedfrom an addition funnel at 101-106° C. over 11 minutes. An additional 20grams of DOWANOL PM were used as a rinse for the addition funnel. Therinse was added to the reaction mixture. The reaction mixture was thenheld at 101° C. for 74 minutes at which time the epoxy equivalent weightwas determined to be greater than 20,000. Then 36 grams of deionizedwater were added and the reaction mixture was held at 100-105° C. for120 minutes. The reaction mixture was then cooled and filled out into aplastic container. The solids of the resin solution were determined tobe 54.61% and the acid number was determined to be 28.0.

COATINGS FORMULATIONS Example 1

[0106] At ambient temperature, a water-based low cure coatingcomposition was made by first adding 36.23 grams of CYMEL® 303 availablefrom Cytec Industries, Inc. to 202.03 grams of Example A. While stirringthe mixture with a Cowles blade, each of the following components wasadded sequentially in one minute intervals: 292.42 grams of FerrophosHRS-3095; 32.58 grams of Shieldex AC3; 0.91 grams of Surfynol 104PA; and2.72 grams of Rheology Modifier QR-708. The resultant mixture was thenstirred with a Cowles blade for 30 minutes. A mild heating occurred. Theinitial viscosity was about 8700 centipoise (RVT Brookfield Spindle 52;5.0 rpm), and grind gauge measurement was 4.5 (Hegman).

Example 2

[0107] At ambient temperature, a solvent-based low cure coatingcomposition was made by first adding 25.70 grams of CYMEL® 303 to 133.73grams of Example C. While stirring the mixture with a Cowles blade,156.02 grams of Ferrophos 3095 were added over one minute followed bythe addition of 17.00 grams of Shieldex AC3 over one minute. Theresultant mixture was then stirred with a Cowles blade for 30 minutes. Amild heating occurred. Upon completion of the stirring, 76.55 grams of1-methoxy-2-propanol were added and the resultant mixture was stirredwith a Cowles blade for 5 minutes. The initial viscosity was about 500centipoise (RVT Brookfield Spindle 52; 50 rpm), and grind gaugemeasurement was 5.0 (Hegman).

Example 3

[0108] A curable coating composition was prepared by stirring 88.1 gramsof 2132 Ferrophos and 7.8 grams of Shieldex AC3 in with 57.5 grams ofEPON®1002F phosphated with phosphoric acid (equivalent ratio ofphosphoric acid to epoxy 1:1.6). After stirring with a Cowles blade for30 minutes, 31.4 grams of Propylene Glycol Monomethyl Ether were addedand mixing was continued. Then 11.0 grams of CYMEL 303, 3.6 grams ofPhosphatized Epoxy (equivalent ratio of phosphoric acid to epoxy of1.6:1), hereinafter “Phosphated Epoxy A” and 1.0 grams ofN-Ethylmorpholine were added. Mixing was continued for another 5minutes.

Example 4

[0109] A curable coating composition was prepared by stirring 43.8 gramsof 2132 Ferrophos and 3.9 grams of Shieldex AC3 in with 27.2 grams ofEPON® 1004F (phosphated with phosphoric acid, equivalent ratio ofphosphoric acid to epoxy of 1.6:1). After stirring with a Cowles for 30minutes, 15.0 grams of Propylene Glycol Monomethyl Ether were added.Then 7.5 grams of CYMEL 11158, 1.8 grams of Phosphatized Epoxy A, and0.5 grams of N-ethylmorpholine were added and mixing was continued for 5minutes.

Example 5

[0110] A curable coating composition was prepared by stirring 2.7 gramsof N-ethylmorpholine in with 46.8 grams Phosphatized EPON® 1004F ofExample 4. Next, 76.4 grams of 2132 Ferrophos and 6.7 grams of ShieldexAC3 were added via stirring. After stirring with a Cowles blade for 30minutes, 27.4 grams of Propylene Glycol Monomethyl Ether-were added.Lastly, 12.7 grams of CYMEL 1158 were added and mixing was continued for5 minutes.

Preparation and-Coating of Substrates

[0111] Two-sided 60G Electrogalvanized Steel (EG) and Zn/Fe two-sidedhot dipped Galvanneal Steel (GA) steel panels were obtained from USXCorporation. Each panel was 15.3 centimeters (cm) wide and 38.1 cm long.The steel panels were subjected to an alkaline cleaning process by sprayin a 20% by volume bath of CHEMKLEEN 163 (CK163) which is available fromPPG Industries, Inc. at a temperature of 60° C. (140° F.) for 60seconds. Alternatively, the steel panels were subjected to an alkalinecleaning process by spray in a 0.85% by weight bath of Parco 338 (P338)which is available from Henkel, Inc. at a temperature of 65° C. (149°F.) for 10 seconds. The panels were removed from the alkaline cleaningbath, rinsed with room temperature deionized water (about 21° C. (70°F.)) for 5 seconds and dried with warm air (about 40° C.).

[0112] Some of the panels were pretreated with NUPAL® 456BZR. Panelscoated with commercially available compositions were prepared with andwithout pretreatment. None of the panels coated with compositionsaccording to the present invention was pretreated.

[0113] After cleaning (and possibly pretreatment in the case of panelscoated with commercially available compositions), the panels were coatedusing wire drawbars and baked at 193° C. (380° F.) for 40 seconds untila peak metal temperature of 143° C. (290° F.) was achieved. Theresulting drawbar type/corresponding dried film thickness values arereported in Table 1. The panels were then quenched with ambienttemperature deionized water and dried prior to testing.

Adhesion and Corrosion Testing

[0114] To determine the adhesion of the coating systems underfabrication conditions, three tests were conducted. For the first twotests, panels coated as described above (without application oflubricant) were subjected to Erichsen adhesion and 160 inch-poundreverse impact tests. A second set of panels was coated with about 1064mg/m² (about 100 mg/ft²) of Quaker 61AUS mill oil and drawn into squarecups 25.4 mm (1 inch) in height and 36.5 mm (1 {fraction (7/16)} inches)along each side. Adhesion performance was evaluated on areas of the cupswhere deformation and strain were the greatest (sides and top/bottomcorners).

[0115] After completion of all three fabrication tests, the panels wereexposed to a phosphate process that would be typical of originalequipment manufacture (OEMs). The phosphate process involves thefollowing steps:

[0116] 1) Spray Clean with CK490MX (2 oz/gal—567 g/10 gal) for 5 minutesat 120° F. and a pressure between 10-20 psi;

[0117] 2) Perform an immersion rinse with warm tap water forapproximately 20 seconds at 120° F.;

[0118] 3) Apply an immersion rinse conditioner (1 g/L) for 1 minute at100° F.;

[0119] 4) Apply an immersion phosphate with CF700 for 2 minutes at 122°F.;

[0120] 5) Perform an immersion rinse with deionized water forapproximately 30 seconds at ambient temperature;

[0121] 6) Perform an immersion seal comprising:

[0122] a) For Europe, use CHEMSEAL™ 19 available from PPG Industries,Inc., adjusted with 10% NH₄OH until pH=4 to 4.5. Apply for approximately1 minute at ambient temperature.

[0123] b) For the United States, use CHEMSEAL 59 available from PPGIndustries, Inc., CS59 (1% v/v, where % v/v stands for volume to volume,i.e., for every 100 mL of solution, there is 1 mL of CS59) adjusted with10% NH₄OH until pH=4 to 4.5. Apply for approximately 1 minute at ambienttemperature.

[0124] 7) Perform a spray bottle final rinse with deionized water. Rinseeach side three times for approximately 5 seconds at ambient 5temperature;

[0125] 8) Dry using warm air; and.

[0126] 9) Bake at 350° F. for 60 minutes.

Tables 1 and 2

[0127] The percentage of area in which complete delamination occurredfor each sample is shown in Tables 1 and 2 below. After the initialadhesion evaluation, cups were placed in corrosion testing (PPG STM-0772based on GM TM-54-26 APG test) for 20 cycles. Relative ratings accordingto the percentage of red rust that formed over the entire tested surfaceof the cup, as well as the degree of white stain, are shown in Table 2.Data from standard high temperature bake controls (with and withoutpretreatment) are shown for comparison.

[0128] In Tables 1 and 2 below, NUPAL is a registered trademark of PPGIndustries, Inc. for metal pretreatment compositions and is described inU.S. Pat. No. 5,858,282 entitled “Aqueous Amine Fluoride NeutralizingComposition for Metal Pretreatments Containing Organic Resin andMethod”. BZ is an abbreviation for BONAZINC™, a trademark of PPGIndustries Ohio, Inc. for zinc-rich coating. MEK is an abbreviation formethyl ethyl ketone. “MEK rubs” is a test for solvent resistance whichentails rubbing a cloth saturated with methyl ethyl ketone back andforth (“double rub”) using normal hand pressure until the coating ismarred. The phosphate test referred to in Table 1 is the ten-stepprocess included in the “Adhesion and Corrosion Testing” section above.TABLE 1 INITIAL INITIAL ADHESION ADHESION INITIAL Erichsen¹ 160 lbADHESION SUBSTRATE Adhesion Rev. Impact² CUPS TESTED COATING % Coating %Coating % Coating (Cleaner type) (PMT Cure) Loss after Loss after Lossafter {Pretreatment} ‘Dry Film MEK Phosphate Phosphate Phosphate if anyThickness’ Rubs Process³ Process³ Process⁴ USX EG BZ3000 100+  <5%  <5%  10% (P338) (254° C.) (Nupal ® 456BZR) ‘3-4 microns’ USX EG BZ3000 20   95 to 100% 95 to 100% 70-80% (P338) (254° C.) (no pretreat) ‘3-4microns’ USX GA BZ3001 100+  <5%  <5%    5% (P338) (232° C.)(Nupal ® 456BZR) ‘3-4 microns’ USX EG BZ3001 100+ 95 to 100% 95 to 100%60-70% (P338) (232° C.) (no pretreat) ‘3-4 microns’ USX EG Example 120-50 <5% <5%  <5% (CK163) (140° C.) (no pretreat) ‘4-5 microns’ USX GAExample 1 100+ <5% <5%    5% (CK163) (140° C.) (no pretreat) ‘4-5microns’ USX EG Example 2 100+ <5% <5%    5% (CK163) (140° C.) (nopretreat) ‘4-5 microns’ USX GA Example 2 100+ <5% <5%    5% (CK163)(140° C.) (no pretreat) ‘4-5 microns’ USX EG Example 3 20-50 50-70%70-90%    5% (P338) (140° C.) (no pretreat) ‘3-4 microns’ USX EG Example4 100+ <5% <5%    5% (P338) (140° C.) (no pretreat) ‘3-4 microns’ USX EGExample 5 100+ <5% <5%    5% (P338) (140° C.) (no pretreat) ‘4-5microns’

[0129] TABLE 2 APG APG SUBSTRATE TESTING TESTING TESTED COATING PANELSCUPS (Cleaner type) (PMT Cure) % RED RUST % RED RUST {Pretreatment} ‘DryFilm (Degree of (Degree of if any Thickness’ White Stain)¹ White Stain)¹USX EG BZ3000 10-30% 80-90% (P338) (254° C.) (Moderate) (Moderate)(Nupal ® 456BZR)² ‘3-4 microns’ USX GA BZ3001  5-20% 20-30% (P338) (232°C.) (Moderate) (Moderate) (Nupal ® 456BZR)² ‘3-4 microns’ USX EG Example1  2-3% 30-40% (CK163) (140° C.) (light) (Moderate) (no pretreat) ‘4-5microns’ USX GA Example 1 <2% 20-30% (CK163) (140° C.) (light)(Moderate) (no pretreat) ‘4-5 microns’ USX EG Example 2  3-5% 30-40%(CK163) (140° C.) (light) (Moderate) (no pretreat) ‘4-5 microns’ USX GAExample 2 <2% 10-15% (CK163) (140° C. ) (light) (light) (no pretreat)‘4-5 microns’ USX EG Example 3  5-30% 20-30% (P338) (140° C.) (light to(moderate) (no pretreat) ‘3-4 microns’ moderate) USX EG Example 4  5-20%20-30% (P338) (140° C.) (light to (moderate) (no pretreat) ‘3-4 microns’moderate) USX EG Example 5  3-5% 25-35% (P338) (140° C.) (light)(moderate) (no pretreat) ‘4-5 microns’ USX EG Example 5 <2%  5-15%(P338) (140° C.) (light) (light) (no pretreat) ‘4-5 microns’

[0130] The data reported in Tables 1 and 2 above show that the coatingcompositions compare very favorably with commercially availablezinc-rich coatings. The panels coated with compositions according to thepresent invention demonstrated excellent adhesion and corrosionresistance properties without metal pretreatment. In contrast to theresults obtained using the composition of the present invention, panelscoated with commercially available coatings did not demonstratesatisfactory adhesion and corrosion resistance without metalpretreatment. In addition to the demonstrated excellent adhesion andcorrosion resistance properties, the compositions of the presentinvention can be cured at lower temperatures than commercially availablecoatings.

Weldability Test

[0131] The coating compositions of the present invention were tested forspot, weldability by coating two steel sheets on both sides withcompositions of the present invention. Each sheet was approximately 2-½inches by 12 inches by 0.030 inches. The sheets were welded togetherrepeatedly—each weld being spaced between ⅜ inches and ½ inches apart.After about 50 welds, the welded sheets were allowed to cool to preventthe sheets from becoming excessively hot. After cooling, another 50welds were administered to the sheet. Testing Methods A and B, used toevaluate the coatings of this invention, involve approximately 1000welds and measure welding parameters A_(min), the welding current neededto form a “minimum nugget” and A_(max), the highest current that can beused without violently ejecting molten metal from the weld(“expulsion”).

[0132] Table 3 below summarizes the results of weldability testing. InTable 3, “lobe width” refers to the difference between A_(min) andA_(max). The “current stepping required” refers to the degree of currentstepping required to maintain a certain safety margin which is definedas the excess current used beyond that needed to form a minimum nuggetduring the test. The parameters used to generate the welding data are asfollows: Weld force=470 pounds; Squeeze time=45 cycles=45/60 sec; Weldtime=9 cycles=9/60 sec; Hold time=5 cycles=5/60 sec; Off time=40cycles=40/60 sec; Rate of Welding=36 welds per minute; and A_(min)=3.6mm diameter. TABLE 3¹ Coating Lobe width lobe width lobe width Currentthickness, Test after 50 after 500 after 950 stepping Example micronsSteel type² method welds welds welds required Comment 1 3.4 GA  A³ 1.2kA 1.5 kA 1.0 kA 0.8 kA A_(min) 7.2 to 8.0 2 3.8 EG A 1.1 kA 1.1 kA 1.1kA 0.2 kA A_(min) 7.2 to 7.4 2 8.1 GA A 1.5 kA 1.2 kA 0.7 kA 1.1 kAA_(min) 6.8 to 7.9 5 4.0 EG B 1.9 kA 1.7 kA 1.1 kA 0.5 kA A_(max) 8.8 to9.3 5 6.8 GA  B⁴ 1.4 kA 1.2 kA 0.9 kA 0.1 kA A_(max) 8.5 to 8.6 Bonazinc3.7 EG A 1.9 kA 1.2 kA 1.3 kA 1.4 kA A_(min) 6.9 to 8.3 3000 Bonazinc3.7 EG A 1.2 kA 0.9 kA 1.6 kA 1.4 kA A_(min) 6.4 to 7.8 3001 #Warminster, Pennsylvania with a starting face diameter of 3/16 inch wereused.

[0133] All welds were successful using Examples 1, 2 and 5 since thehigh amperage welding current passed through the sheets. There were nocases in which the welding tips became fouled to the extent that currentflow was prevented. Because (1) the amount of current stepping requiredto maintain a welding safety margin over 1000 welds is smaller thancommercially used controls and (2) it is no larger than some examplesutilized in commercial automobile assembly, all of the examples arejudged to be weldable for the purposes of repetitive automotive spotwelding.

What is claimed is:
 1. A curable coating composition comprising a. aresinous binder comprising: i. a reaction product of an epoxy-containingpolymer with a compound containing phosphorus acid groups, the reactionproduct having reactive functional groups, ii. a curing agent havingfunctional groups reactive with the functional groups of (i); b. anelectroconductive pigment dispersed in (a) such that the weight ratio ofb to (i) plus (ii) is within the range of 0.5 to 9.0:1, the curablecoating composition being characterized such that when it is depositedand cured on a metal substrate, the cured coating is weldable.
 2. Thecurable coating composition according to claim 1 in which theepoxy-containing polymer is a polyglycidyl ether of a polyhydric phenol.3. The curable coating composition according to claim 2 where thepolyhydric phenol is Bisphenol A.
 4. The curable coating compositionaccording to claim 1 wherein the molecular weight of theepoxy-containing polymer is 220-4500 based on epoxy equivalentmultiplied by the epoxy functionality.
 5. The curable coatingcomposition according to claim 1 wherein the compound containingphosphorus acid groups is selected from the group comprising phosphoricacid, a phosphonic acid, and phosphorous acid.
 6. The curable coatingcomposition according to claim 1 wherein the equivalent ratio of thecompound containing phosphorus acid groups to epoxy-containing polymeris within the range of 0.5 to 3.5:1.
 7. The curable coating compositionaccording to claim 1 wherein the functional groups of (i) are hydroxylgroups or hydroxyl and epoxy groups.
 8. The curable coating compositionaccording to claim 1 wherein the curing agent is selected from the groupcomprising aminoplast resins, polyisocyanates, polyacids, organometalliccomplexed materials, polyamines, and polyamides.
 9. The curable coatingcomposition according to claim 8 wherein the curing agent is anaminoplast.
 10. The curable coating composition according to claim 1wherein said electroconductive pigment is selected from the groupcomprising zinc, aluminum, iron, graphite, diiron phosphide, tungsten,stainless steel, and mixtures thereof.
 11. The curable coatingcomposition according to claim 1 wherein the weight percent of (i) basedon the total weight of resinous binder is from 50 to 90 percent.
 12. Thecurable coating composition according to claim 1 wherein the weightpercent of (ii) based on the total weight of resinous binder is from 10to 50 percent.
 13. The curable coating composition according to claim 1wherein the weight percent of (a) based on the total weight of (a) plus(b) is from 10 to 55 percent.
 14. The curable coating compositionaccording to claim 1 wherein the weight percent of (b) based on thetotal weight of (a) plus (b) is from 45 to 90 percent.
 15. The curablecoating composition according to claim 1 further comprising corrosionresistant pigments.
 16. An aqueous-based curable coating compositioncomprising: a. a resinous binder comprising i. a reaction product of anepoxy-containing polymer with a compound containing phosphorus acidgroups, the reaction product having reactive functional groups, ii. acuring agent having functional groups reactive with the functionalgroups of (i); b. an electroconductive pigment dispersed in (a) suchthat the weight ratio of b to (i) plus (ii) is within the range of 0.5to 9.0:1; and c. water, the coating composition being characterized suchthat when it is deposited and cured on a metal substrate, the curedcoating is weldable.
 17. The coating composition according to claim 16further comprising stabilizers, dispersants, and thickeners.
 18. Thecoating composition according to claim 17 wherein thestabilizer/dispersant is potassium tripolyphosphate.
 19. The coatingcomposition according to claim 16 further comprising an amine.
 20. Thecoating composition according to claim 16 further comprising corrosioninhibiting pigments.
 21. An organic solvent-based curable coatingcomposition comprising: a. a resinous binder comprising i. a reactionproduct of an epoxy-containing polymer with a compound containingphosphorus acid groups, the reaction product having reactive functionalgroups, ii. a curing agent having functional groups reactive with thefunctional groups of (i); b. an electroconductive pigment dispersed in(a) such that the weight ratio of b to (i) plus (ii) is within the rangeof 0.5 to 9.0:1; and c. an organic solvent, the curable coatingcomposition being characterized such that when it is deposited cured ona metal substrate, the cured coating is weldable.
 22. The coatingcomposition according to claim 21 further comprising corrosion resistantpigments.
 23. The coating composition according to claim 21 furthercomprising an amine.
 24. The coating composition according to claim 21wherein the amine is N-methyl or N-ethyl morpholine.
 25. A process forcoating a continuous metal sheet comprising: a. applying directly to themetal sheet shortly after it is formed and at a temperature of 20 to 150° C., a curable coating composition comprising: i. a resinous bindercomprising (A) a reaction product of an epoxy-containing polymer with acompound containing phosphorus acid groups, the reaction product havingreactive functional groups, (B) a curing agent having functional groupsreactive with the functional groups of (A); ii. an electroconductivepigment dispersed in (i) such that the weight ratio of (ii) to (A) plus(B) is within the range of 0.5 to 9.0:1, the curable coating compositionbeing characterized such that when it is deposited and cured on a metalsubstrate, the cured coating is weldable; and b. drying the coatingcomposition on the metal sheet.
 26. The process according to claim 25wherein the metal sheet is selected from the group comprising ferrousmetals, non-ferrous metals, and combinations thereof.
 27. A process forcoating a continuous metal sheet comprising: a. unwinding the metalsheet from a metal coil and passing the metal sheet in a substantiallycontinuous manner through a cleaning station, a coating station, and acuring station; b. applying to the metal sheet at the coating station acurable coating composition comprising: i. a resinous binder comprising:(A) a reaction product of an epoxy-containing polymer with a compoundcontaining phosphorus acid groups, the reaction product having reactivefunctional groups, (B) a curing agent having functional groups reactivewith the functional groups of (A); ii. an electroconductive pigmentdispersed in (i) such that the weight ratio of (ii) to (A) plus (B) iswithin the range of 0.5 to 9.0:1; and c. curing the coating compositionapplied to the metal sheet in step (b) as the coated metal sheet passesthrough the curing station.
 28. A process according to claim 27 furthercomprising galvanizing the metal sheet and then immediately performingthe step of applying the curable coating composition.